libwebp: fix webpmux lib name for cmake linking

When using CMake the `<prefix>/usr/share/WebP/cmake/WebPConfig.cmake` gets
used to get the names of the libraries to link against.

Since version 1.2.1 of libwebp, libwebpmux is on by default.
This causes a linker error because the linker arg should be `-lwebpmux`
instead of `-llibwebpmux`.

This is easily fixable by correcting the generation of the WebPConfig.cmake
file to append `webpmux` when this lib is enabled.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>

libs/libwebp/patches/001-fix-cmake-webpmux-linking.patch

@@ -0,0 +1,55 @@
+--- a/CMakeLists.txt
++++ b/CMakeLists.txt
+@@ -471,19 +471,18 @@ endif()
+ 
+ if(WEBP_BUILD_LIBWEBPMUX)
+   parse_makefile_am(${CMAKE_CURRENT_SOURCE_DIR}/src/mux "WEBP_MUX_SRCS" "")
+-  add_library(libwebpmux ${WEBP_MUX_SRCS})
+-  target_link_libraries(libwebpmux webp)
+-  target_include_directories(libwebpmux
++  add_library(webpmux ${WEBP_MUX_SRCS})
++  target_link_libraries(webpmux webp)
++  target_include_directories(webpmux
+                              PRIVATE ${CMAKE_CURRENT_BINARY_DIR}
+                                      ${CMAKE_CURRENT_SOURCE_DIR})
+-  set_version(mux/Makefile.am libwebpmux webpmux)
+-  set_target_properties(libwebpmux
++  set_version(mux/Makefile.am webpmux webpmux)
++  set_target_properties(webpmux
+                         PROPERTIES PUBLIC_HEADER
+                                    "${CMAKE_CURRENT_SOURCE_DIR}/src/webp/mux.h;\
+ ${CMAKE_CURRENT_SOURCE_DIR}/src/webp/mux_types.h;\
+ ${CMAKE_CURRENT_SOURCE_DIR}/src/webp/types.h;")
+-  set_target_properties(libwebpmux PROPERTIES OUTPUT_NAME webpmux)
+-  list(APPEND INSTALLED_LIBRARIES libwebpmux)
++  list(APPEND INSTALLED_LIBRARIES webpmux)
+   configure_pkg_config("src/mux/libwebpmux.pc")
+ endif()
+ 
+@@ -497,7 +496,7 @@ if(WEBP_BUILD_GIF2WEBP)
+                         exampleutil
+                         imageioutil
+                         webp
+-                        libwebpmux
++                        webpmux
+                         ${WEBP_DEP_GIF_LIBRARIES})
+   target_include_directories(gif2webp PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/src)
+   install(TARGETS gif2webp RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+@@ -514,7 +513,7 @@ if(WEBP_BUILD_IMG2WEBP)
+                         imagedec
+                         imageioutil
+                         webp
+-                        libwebpmux)
++                        webpmux)
+   target_include_directories(img2webp PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/src)
+   install(TARGETS img2webp RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+ endif()
+@@ -563,7 +562,7 @@ if(WEBP_BUILD_WEBPMUX)
+   parse_makefile_am(${CMAKE_CURRENT_SOURCE_DIR}/examples "WEBPMUX_SRCS"
+                     "webpmux")
+   add_executable(webpmux ${WEBPMUX_SRCS})
+-  target_link_libraries(webpmux exampleutil imageioutil libwebpmux webp)
++  target_link_libraries(webpmux exampleutil imageioutil webpmux webp)
+   target_include_directories(webpmux PRIVATE ${CMAKE_CURRENT_BINARY_DIR}/src)
+   install(TARGETS webpmux RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
+ endif()